Semiconductor laser device

ABSTRACT

There is provided a semiconductor laser device comprising a lower optical confinement layer 31, an active layer 3 and an upper optical confinement layer 34, said active layer 3 being inserted between said upper and lower optical confinement layers 31 and 34, characterized in that an electron injection path (along an n-type electrode 10→the upper optical confinement layer 34→the active layer 3) for injecting electrons into the active layer 3 by way of the optical confinement layers and a hole injection path (along p-type electrodes 6→the p-type InP layer 5→the active layer 3) for injecting holes into the active layer 3 without passing through the optical confinement layers are formed therein. With such an arrangement, electrons are injected into the active layer by way of the optical confinement layers, whereas holes are injected into the active layer without passing through the optical confinement layers. Therefore, the device shows a remarkable improvement in the modulation response and modulation bandwidth.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a semiconductor laser device to be suitablyused as transmission light source in the field of opticaltelecommunications and optical instrumentation.

2. Prior Art

Data transmission systems having a transmission rate of 10 Gb/sec. or sohave been proposed for large capacity trunk lines of the cominggeneration in the field of optical telecommunications. In thisconnection, efforts have been paid to exploit the potential of solitontransmission as it serves for optical data transmission withoutaffecting the dispersion characteristics of the optical fibers involved.

Semiconductor laser devices to be used for soliton transmission arerequired to show a high speed modulation capability for frequencies of20 GHz and above in order to generate ultrashort pulses of light with apulse width not greater than 1×10⁻¹² sec. (1 pico-sec.).

Paper 1 listed below discusses some of the results of theoreticalresearches made on the properties of semiconductor laser devicesconcerning light wave modulation, using rate equations [1a] and [1b]below for photon density S and carrier density N in an active layer.

Paper 1: IEEE J. Quantum Electronics, Vol. QU-22, p. 833-844, 1086.

    dS/dt=ΓG(S,N)-S/τ.sub.p                          [ 1a]

and

    dN/dt=J(t)-N/τ.sub.n -G(S,N)                           [1b]

where,

τ_(p) : the photon lifetime in a laser cavity,

τ_(n) : the damping time constant of the carrier,

G(S, N): the laser gain,

J(t): the density of injected carriers including the modulationcomponent and

Γ: optical confinement factor.

The modulation response of a semiconductor laser device can bedetermined by solving rate equations [1a] and [1b] and its highestmodulable cutoff frequency is obtained by using equation [2] below.

    f.sub.m =2π2.sup.1/2 /K                                 [2]

where, K is expressed by equation [3] below.

    K=4π.sup.2 (τ.sub.p +ε/g.sub.o)             [3]

where g_(o) and ε respectively represent the linear differential gainand the saturation constant of gain G(S, N) above when it is expressedin terms of threshold carrier density N_(t) as shown by equation [4]below.

    G(S,N)-[g.sub.o S/(1+εS)](N-N.sub.t)               [4]

By studying the above equations, it is evident that the value of K needsto be reduced in order to expand the modulation bandwidth of asemiconductor laser device by increasing frequency fm of thesemiconductor laser device and, by turn, it is necessary to reduce p,increase g_(o) or reduce ε in order to reduce K.

Known techniques for reducing τ_(p) include reducing the length of thecavity and/or the reflectivity of the facet of the semiconductor lasercavity, whereas it is known that g_(o) can be increased by doping theactive layer to turn it into a p-type layer.

However, the values of g_(o) and ε are invariable and τ_(p) can easilyencounter a lower limit because it is correlated with the thresholdcurrent density when the semiconductor laser device is prepared by usinga single and same semiconductor material such as GaInAsP. Thus, theabove identified techniques are not feasible to reduce the value of Kfor such a semiconductor laser device.

A promising technique is the use of a multiple or single quantum wellstructure for the active layer in order to multiply the currentlyavailable value of g_(o) by two to three times.

On the other hand, it, is pointed out in Paper 2 below that a quantumwell structure can degrade the modulation response of a semiconductorlaser device and therefore the time required for carriers to be injectedinto a quantum well structure is inevitably prolonged because of thespecific properties of the quantum well structure.

Paper 2: 17th European Conference on Optical Communication, paper Tu.A4.3, Paris, France, September 1991.

Now, some of the problems pointed out in Paper 2 above will besummarized below.

The rate equations for a semiconductor laser device having a quantumwell structure can be obtained by modifying equations [1a] and [1b] asshown below.

    dS/dt=ΓG(S,N.sub.w)-S/τ.sub.p                    [ 5a],

    dN.sub.b /dt=J.sub.b (t)-N.sub.b /n-N.sub.b /τ.sub.r +N.sub.w /τ.sub.e (V.sub.w /V.sub.b)                           [5b]

and

    dN.sub.w /dt=J.sub.w (t)-N.sub.w /τ.sub.e +N.sub.b /τ.sub.r (V.sub.b /V.sub.w)-G(S,N.sub.w)                           [5c]

where,

N_(w) : the density of carriers in the quantum well layer,

N_(b) : the density of carriers in the barrier layer,

V_(w) : the volume of the quantum well layer,

V_(b) : the volume of the barrier layer,

J_(w) (t): the density of carriers directly injected into the quantumwell layer,

J_(b) (t): the density of carriers injected into the barrier layer,

τ_(r) : the time required for carriers in the barrier layer to becaptured by the quantum well layer (which depends on the time requiredfor carriers to run through the barrier layer or the optical confinementlayer) and

τ_(e) : the elapsed time (extent) for carriers to be thermally emittedfrom the quantum well layer, so-called thermionic emission.

A conventional semiconductor laser device having a multiple quantum wellstructure modifies the density of carriers ΔJ_(b) injected into thebarrier layer by reducing the density of carriers injected into thequantum well layer ΔJ_(w) to zero but the modulation response of asemiconductor laser device, or the response of photon density S toΔJ_(b), obtained by using such a modulation technique of modulating thecarrier density is not desirable because of the reasons as describedbelow by referring to equations [5a], [5b] and [5c].

Firstly, there occurs deterioration in the cutoff frequency as the valueof τ_(r) increases because the modulation response of the photon densityS to the carrier density ΔJ_(b) is directly proportional to (1+jWr),where Wr=τ_(r) ⁻¹.

Secondly, the square of the relaxation frequency is inverselyproportional to the emission parameter of carriers (α) as defined byequation [6] below.

    α=1+(τ.sub.r /τ.sub.e)                       [6]

Thirdly, an increase in the value of τ_(r) and decrease in the value ofτ_(e) may often be observed, when the optical confinement layer and thequantum well layer are respectively made to be greater than 1,000 Å andsmaller than 50 Å in an attempt to achieve a high output level of thedevice.

When τ_(r) increases while τ_(e) decreases, the cutoff frequency of asemiconductor laser device is lowered because of the increase in thevalue of α attributable to the increased τ_(r) and decreased τ_(e)values.

Thus, the modulation speed of a conventional quantum well typesemiconductor laser device is limited by the slow response of carriersto obstruct any quick modulation of injection current.

This is a problem that surface-emission type semiconductor laser devicescommonly experience when they are used for optical interconnection orparallel transmission.

Some of the problems of surface-emission type semiconductor laserdevices are pointed out in Papers 3 and 4 below.

Firstly, while the film thickness controllability of a semiconductormultilayer film can be improved depending on temporary film formation ina same grown junction device, there occurs a phenomenon that theresistance of the semiconductor multilayer film becomes high on thep-electrode side. This phenomenon can be observed particularly in InPtype multilayer films.

Secondly, a high-speed modulation becomes impossible when a pn junctiontype current blocking layer is used in the formation of a cavity for aburied structure because of a large parasitic capacity generated there.

Thirdly, if a device is made capable of high-speed modulation, themodulation response of the device can easily be degraded because of apoor thermal dispersion capability of its mesa-type active layer and alarge current loss and an increased threshold current level can appear,because the current in the active layer becomes concentrated on thep-electrode side to reduce the current density at the center of thelayer and therefore the overlapping area of the injected current and thephotoelectric field.

Paper 3: 48th Device Research Conference., 5A-31, June, 1990.

Paper 4: IEEE J. Quantum Electron., QE-24, No. 9, pp. 1845-1855,September 1986.

Paper 5: 48th Device Research Conference., Post Deadline Paper 5B-2,June, 1990.

Paper 6: 49th Device Research Conference., Post Deadline Paper 3A-8,June, 1990.

SUMMARY OF THE INVENTION

In view of the above identified technological problems, it is thereforean object of the present invention to provide a semiconductor laserdevice having a high-speed modulation capability.

According to the invention, the above object is achieved by providing asemiconductor laser device comprising a lower optical confinement layer,an active layer and an upper optical confinement layer of said activelayer being inserted between said upper and lower optical confinementlayers, characterized in that an electron injection path for injectingelectrons into the active layer by way of the optical confinement layersand a hole injection path for injecting holes into the active layerwithout passing through the optical confinement layers are formedtherein.

When a semiconductor laser device according to the invention is of aquantum well type, the lower optical confinement layer, the active layerand the upper optical confinement layer may preferably be formed into amesa section having an upper side covered by a p-type layer and lateralsides covered by an n-type layer.

When a semiconductor laser device according to the present invention hasan above described configuration, the active layer and the barrier layermay be preferably made of a quantum well layer, whereas the lower andupper optical confinement layers may be preferably made of a singlesemiconductor layer.

Such a quantum well type semiconductor laser device may preferably havean n-electrode arranged on the p-type layer covering the upper side ofthe mesa section and p-electrodes arranged on the respective lateralsides of the mesa section, as it comprises as basic components anelectron injection path for injecting electrons into the active layer byway of the optical confinement layers and a hole injection path forinjecting holes into the active layer without passing through theoptical confinement layers.

Since a quantum well type semiconductor laser device according to theinvention, unlike a comparable conventional device, directly modifiesthe carrier density in the active layer (quantum well layer), or J_(b),by establishing a relationship of J_(b) =0, the modulation response ofthe laser device (the response of the photon density S to ΔJ_(w)) willbe theoretically improved in a following manner.

Firstly, the modulation characteristics of such a device show aremarkable improvement as compared with those of a conventional devicebecause of nonexistence of a roll off term of the first degree, or(1+jWr)⁻¹.

Secondly, the modulation characteristics of such a device are alsoremarkably improved because the relaxation frequency increases bydecreasing a close to one to allow a wide quantum well layer and narrowoptical confinement layers.

The modulation responses of such a quantum well type semiconductor laserdevice will be further improved if electrons are injected into thequantum well layer by way of the optical confinement layers and thebarrier layer and holes are injected directly into the quantum welllayer without passing through the optical confinement layers and thebarrier layer because the carrier density in the quantum well layer isdirectly modulated.

If a semiconductor laser device according to the invention is of asurface emission type, the active layer formed on the lower opticalconfinement layer may preferably take a mesa shape and the upper side ofthe mesa section may be covered by a p-type layer and the upper opticalconfinement layer located thereabove, while the lateral sides of themesa section may be covered by an n-type layer.

When a semiconductor laser device according to the invention isconfigured in an above described manner, the lower and upper opticalconfinement layers may be made of a semi-insulating multilayer film.

Such a surface emission type semiconductor laser device may preferablyhave an n-electrode arranged on the p-type layer covering the upper sideof the mesa section and p-electrodes arranged on the respective lateralsides of the mesa section, as it comprises as basic components anelectron injection path for injecting electrons into the active layer byway of the optical confinement layers and a hole injection path forinjecting holes into the active layer without passing through theoptical confinement layers.

With such a surface-emission type semiconductor laser device, when lessmobile holes are injected into the active layer without passing throughthe highly resistive optical confinement layers, there will be no straycapacitance such as pn junction and therefore a high-speed response willbe realized, because the current injection into the active layer iscarried out evenly to reduce the DC resistance of the device and, at thesame time, the n-electrode and the p-electrodes arranged at the abovedescribed respective positions on the substrate are not oppositelylocated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a first embodiment ofsemiconductor laser device of the invention, which is a quantum welltype semiconductor laser device.

FIG. 2 is a graph showing the modulation response of a quantum well typesemiconductor laser device according to the invention and obtained by asimulation.

FIGS. 3 and 4 are respectively a cross-sectional view and a plane viewof a second embodiment of semiconductor laser device of the invention,which is a surface emission type semiconductor laser device.

THE BEST MODES FOR CARRYING OUT THE INVENTION

FIG. 1 schematically illustrates a first embodiment of semiconductorlaser device according to the invention, which is a quantum well typesemiconductor laser device.

Such a quantum well type semiconductor laser device may typically beprepared in a following manner.

To begin with, a 1,000 Å thick nondoped InP layer 2, a 300 Å thickInGaAsP lower optical confinement layer 31 having a band gap wavelengthλ_(g) =1.1 μm, a 900 Å thick active layer 3 including a multiple quantumwell layer, a 300 Å thick InGaAsP upper optical confinement layer 34having a band gap wavelength λ_(g) =1.1 μm, a 1,000 Å thick nondoped InPlayer 4 are sequentially formed on a semiconductor InP semi-insulatingsubstrate 1 in a first metalorganic chemical vapor deposition (MOCVD)process.

The above active layer 3 is in fact a multilayer structure formed byalternately arranged six 100 Å thick InGaAsP barrier layers with a bandgap wavelength λ_(g) =1.1 μm and seven 50 Å thick InGaAsP opticalconfinement layers 33 with a band gap wavelength λ_(g) =1.1 μm.

Then, an SiO₂ film is formed on the nondoped InP layer 4 by means of aplasma CVD technique and processed to produce a stripe-shapedstop-etching mask by photolithography.

Thereafter, the areas not covered by the mask or the lateral sides ofthe layers from the nondoped InP layer 4 down to the InP semi-insulatingsubstrate 1 are chemically etched to produce a mesa section.

Then, a p-type InP layer 5, a p-type InGaAsP layer 6 and asemi-insulating InP layer 7 are formed on the lateral sides of or aroundthe mesa section in a second MOCVD process in such a manner that thelateral sides of the mesa section is covered by the p-type InP layer 5whereas the p-type InGaAsP layer 6 and the semi-insulating InP layer 7are sequentially formed around the mesa section.

Subsequently, when the SiO₂ mask is removed, an n-type InP layer 8 andan n-type InGaAsP layer 9 are sequentially formed on the upper surfaceof the prepared semiproduct to cover the nondoped InP layer 4 throughthe p-type InGaAsP layer 6 in a third MOCVD process.

Finally, an n-type electrode 10 is formed on the n-type InGaAsP layer 9and, as a mesa is formed by chemical etching, using the electrode as amask, the p-type InGaAsP layer 6 is exposed as a stop-etching layer onthe lateral sides, p-type electrodes 11 are formed on the respectivesurfaces of the layer.

With a quantum well type semiconductor laser device prepared in an abovedescribed manner, electrons are injected sequentially into the upperoptical confinement layer 34, the barrier layer 32 and the quantum welllayer 33 though a channel passing through the n-type InGaAsP layer 9,the n-type InP layer 8, the nondoped InP layer 4, whereas holes areinjected from the buried layer, or the p-type InP layer 9, on thelateral sides of the mesa section directly into the quantum well layer33 without passing through the lower optical confinement layer 31, theupper optical confinement layer 34, the barrier layer 32 and otherlayers.

FIG. 2 shows a graph showing the modulation response of a quantum welltype semiconductor laser device according to the invention and obtainedby a simulation along with those of a conventional device using aconventional modulation technique.

The value of τ_(r) is defined by the thickness of the opticalconfinement layers and the mobility of holes and assumed to beapproximately 50 psec.

It will be obvious from FIG. 2 that the modulation response of of aconventional device shows a roll-off phenomenon that at thecorner-frequency of f_(c) =(2πτ_(r))⁻¹ ·3.2 GHz, which considerablyreduce the modulation speed.

This will be explained by the fact that, in a conventional device,electrons having a small effective mass and a long diffusion distancereadily diffuse through the wide optical confinement layers and passover the barrier layer, whereas holes having properties opposite tothose of electrons take a long time to diffuse through the opticalconfinement layers and pass over the barrier layer.

Contrary to this and as described earlier, in a quantum wellsemiconductor laser device according to the present invention, sinceelectrons are injected through the optical confinement layers and thebarrier layer into the quantum well layer, while holes are injecteddirectly into the quantum well layer without passing through the opticalconfinement layers and the barrier layer, the carrier density within thequantum well layer is directly modulated to improve the modulationresponse of the device.

FIGS. 3 and 4 illustrates a second embodiment of the invention, which isa surface emission type semiconductor laser device.

Such a surface emission type semiconductor laser device may typically beprepared in a following manner.

To begin with, a semi-insulating multilayer film (multilayer reflectionfilm) 42, a p-type doped InP lower clad layer 43, a nondoped InGaAsPactive layer 44, a nondoped InP upper clad layer 45 are sequentiallyformed on a semiconductor InP semi-insulating substrate 1 by epitaxialgrowth in a first MOCVD process.

The above semi-insulating multilayer film 42 is formed by alternatelyarranging thirty 2,000 Å thick InGaAsP layers with a band gap wavelengthλ_(g) =1.4 μm and thirty 2,000 Å thick InP layers with a band gapwavelength λ_(g) =1.4 μm.

The p-type doped InP lower clad layer 43 is 1 μm thick and doped with Znto a concentration of approximately 5×10¹⁷ cm⁻³.

The nondoped InGaAsP active layer 44 is 0.6 μm thick and has a band gapwavelength λ_(g) =1.5 μm.

The nondoped upper clad layer 45 is 0.5 μm thick.

Then, an SiO₂ film is formed on the nondoped InP upper clad layer 45 byplasma CVD technique and processed by photolithography to show acircular form having a diameter of 4 μm so that it may be used as anetching-resist mask.

Thereafter, the areas not covered by the mask or the surrounding areasfrom the nondoped InP layer 45 down to the InP semi-insulating substrate41 are chemically etched to produce a circular mesa section.

Then, a p-type InP surrounding lateral layer 46, a p-type InGaAsPcontact layer 47 and a semi-insulating InP layer 48 for currentconfinement are formed on the upper surface and the lateral sides of oraround the circular mesa section in a second MOCVD process using theSiO₂ film as a mask for selective growth in such a manner that thecircular mesa section is covered by these layers.

The p-type InP surrounding lateral layer 46 is doped with Zn to aconcentration of approximately 5×10¹⁷ cm⁻³.

The nondoped InGaAsP contact layer 47 is doped with Zn to aconcentration of approximately 1×10¹⁹ cm⁻³ and has a band gap wavelengthλ_(g) =1.3 μm.

The semi-insulating InP layer 48 is doped with Fe.

Subsequently, when the SiO₂ mask is removed, an n-type InP layer 49 andan n-type semi-insulating multilayer film (multilayer reflection film)50 are sequentially formed on the upper surface of the preparedsemiproduct to cover the nondoped InP layer 45 through thesemi-insulating InP layer 48 in a third organometallic vapor depositionprocess.

The n-type InP layer 49 shows a carrier density of approximately 1×10¹⁸cm⁻³.

The n-type semi-insulating multilayer film 50 has a structuresubstantially same as that of the semi-insulating film 42 except that ithas a carrier density of approximately 1×10¹⁸ cm⁻³ and is doped ton-type.

Finally, an n-type electrode 91 is formed on the n-type semi-insulatingmultilayer film 50 and, as a circular mesa is formed by chemicaletching, using the electrode as a mask, the p-type InGaAsP contactlayers 47 is exposed as an etching prohibiting layer, p-type electrodes52 are formed on the respective surfaces of the p-type InGaAsP contactlayer.

With a surface emission type semiconductor laser device prepared in anabove described manner, the electric current injected from the p-typeelectrodes 52 are introduced into the non-doped InGaAsP active layer 44through its lower surface and the lateral sides by way of the p-typedoped lower clad layer 43 and the p-type InP surrounding lateral cladlayer 46 without passing through the semi-insulating multilayer film 42.

Consequently, the electric current is not only evenly injected into theactive layer 44 but also does not lose the high-speed response of thedevice, because it is confined by the semi-insulating InP layer 48 inthe device where the n-type electrode 51 and the p-type electrodes 52are not oppositely arranged.

The optical output of an above described surface emission typesemiconductor laser device is emitted along a direction indicated by anarrow in FIG. 3.

The hole injection path as disclosed above is applicable to anysemiconductor laser devices other than the quantum well and the surfaceemission types having a configuration different from those of theembodiments provided that an n-type electrode and p-type electrodes arearranged on the upper surface of a semiconductor wafer.

The semiconductor substrate (semi-insulating substrate) of asemiconductor laser device according to the present invention is notlimited to an InP substrate but any other known substrates includingGaAs substrates may also appropriately be used.

The active layer to be used for the purpose of the present invention maybe either of a single quantum well type or a multiple quantum well typeand the optical confinement layers may have an SCH (Separate ConfineHeterostructure) or GIN-SCH (Graded Index-Separate ConfinementHeterostructure) structure.

The remaining layers of a semiconductor laser device according to thepresent invention may appropriately include III-V group p-type layerscontaining compound semiconductor alloys, n-type layers, doped layers,nondoped layer and any combinations of them.

The n-type and p-type electrodes to be used for the purpose of thepresent invention may be appropriately selected from those of any knowntypes.

[Industrial Applicability]

Since a semiconductor laser device according to the invention comprisesa lower optical confinement layer, an active layer and an upper opticalconfinement layer, said active layer being inserted between said upperand lower optical confinement layers and is characterized in that anelectron injection path for injecting electrons into the active layer byway of the optical confinement layers and a hole injection path forinjecting holes into the active layer without passing through theoptical confinement layers are formed therein, evidently electrons areinjected into the active layer by way of the optical confinement layerswhile holes are directly injected into the active layer without passingthrough the optical confinement layers to remarkably improve themodulation response and the modulation speed of the device.

A semiconductor laser device as disclosed by the present invention canbe usefully and effectively used as a optical source for opticaltransmission in the field of optical telecommunications and opticalinstrumentation because of its high-speed modulation capability.

What is claimed is:
 1. A semiconductor laser device comprising a loweroptical confinement layer, an active layer and an upper opticalconfinement layer, said active layer being positioned between said upperand lower optical confinement layers, characterized in that an electroninjection path for injecting electrons into the active layer by way ofthe optical confinement layers and a hole injection path for injectingholes directly into the active layer substantially without passingthrough the optical confinement layers are formed therein and aninsulating layer is formed between said electron injection path and saidhole injection path.
 2. A semiconductor laser device according to claim1, wherein the lower optical confinement layer, the active layer and theupper optical confinement layer are formed to a mesa section having anupper side covered by a n-type layer and lateral sides covered by anp-type layer.
 3. A semiconductor laser device according to claim 2,wherein the active layer and the barrier layer are made of a quantumwell layer.
 4. A semiconductor laser device according to claim 3,wherein the bandgap of the quantum well layer is longer than the opticalconfinement layers.
 5. A semiconductor laser device according to claim2, wherein the lower and upper optical confinement layers are made of asingle semiconductor layer.
 6. A semiconductor laser device according toclaim 2, wherein an n-electrode is arranged on the n-type layer coveringthe upper side of the mesa section and p-electrodes are arranged on thep-type layer covering lateral sides of the mesa section.
 7. Asemiconductor laser device according to claim 1, wherein a lowerreflection film and an upper reflection film are provided under theupper sides of said active layer.
 8. A semiconductor laser deviceaccording to claim 7, wherein the n-electrode is arranged on upper sideof the upper optical confinement layer and the p-electrodes are arrangedon the upper sides of the n-type layer covering the respective lateralsides of the mesa section.
 9. A semiconductor laser device according toclaim 7, wherein the lower reflection film is made of a non-dopedmultilayer film.
 10. A semiconductor laser device according to claim 7,wherein the upper reflection film consists of an n-type semiconductormultilayer film.